Patent Document 1 discloses a vehicle including an internal combustion engine operated using gas fuel such as CNG (Compressed Natural Gas). When the engine of this vehicle is in idle operation, the opening degree of a throttle valve arranged in an intake passage is controlled to a first opening degree. If deceleration is requested when the vehicle runs, gas fuel supply to the engine is prohibited and the opening degree of the throttle valve is controlled to be smaller than the first opening degree. This increases negative intake pressure at the downstream side of the throttle valve in the intake passage, thus increasing the booster pressure of the booster device. As a result, the booster device appropriately boosts the manipulation force of a brake pedal applied by the driver. In this case, the smaller the pressure at the downstream side of the throttle valve in the intake passage, the greater the negative intake pressure. The booster pressure is a value based on the difference between the negative intake pressure and the atmospheric pressure.
A vehicle known as one such vehicle having an internal combustion engine operated using gas fuel estimates the negative intake pressure and, when the estimate becomes smaller than or equal to a reference value, starts a negative pressure recovery procedure by which the booster pressure is increased. The greater the engine speed, the more likely the negative intake pressure is to be great. The smaller the opening degree of the throttle valve, the more likely the negative intake pressure is to be great. The opening degree of the throttle valve substantially corresponds to the depression amount of an accelerator pedal. Therefore, the negative intake pressure may be estimated using the depression amount of the accelerator pedal, instead of the opening degree of the throttle valve.
However, if the negative intake pressure is estimated and a timing for starting the negative pressure recovery procedure is determined through comparison between the estimate and a reference value, such timing is likely to vary if estimation accuracy of the negative intake pressure is small.